Hi,
first, are you able to select the OBJECT node with its @id "objectB" ? that's your first step, you save this node into a reference. Use the DataMaker reference browser to check the content of all your references, launch it via the menu PlayMaker/addon/DataMaker/Xml/Reference browser
Once you have this node in reference, you use the action XmlSetNodeProperties, by passing the reference, and you set one property, in this case an attribute so the property would be "@colour".
Let me know where you are in this process.
Bye,
Jean